Well, approximately 5 billion years ago an enormous cloud of interstellar hydrogen began to contract under the influence of its own gravity. As the slow-moving hydrogen atoms collided with each other more and more frequently, their aggregate velocity rose increasing the temperature. Density (and thus temperature) was highest at the center of the cloud. Eventually the collision velocities crossed the threshold necessary to initiate the classic hydrogen-to-helium stellar fusion process. The process liberated sufficient heat to counteract further gravitational collapse, and eventually a balance was achieved between the inward pressure of gravity and the outward pressure of ...